Unfortunately this looks just to be an American thing.
At the bottom of the terms there are 2 sections. The first says just for America and the Second for everyone else also.
So i guess the first is the scavenger hunt and the second the map pack offer.

Artifact isn't valid for me, it says I missed my chance to enter that code?

I should clarify that I entered the first code EXPLORE just today and it worked, so I'm just a little confused by this. I haven't read anything about them expiring...

"Secret Codes: One (1) Secret Code will be released per Secret Code period ("Secret Code Period") in the chart below. The Secret Code released in the first Secret Code Period may be entered at any time. The Secret Codes released in Secret Code Periods 2 through 4 must be entered during the same Period they are released."

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